[morzh]

Terry

To answer the two last questions:

All bypass caps are as specified by the sch and are bough from Mouser, mostly Panasonic. Adding extra on top of them (tried) changes exactly zilch.
Yes did replace the 1st Audio, have just bought cheaply bunch of good 78 and 77 tubes (needed a couple but at $3 on average bought 4 of each). Same result.

Yes, rectification is not the way out for me, as I know that the other radio does not seem to exhibit that so it should work the way it is (or the way it should be in case something is botched). Plus, it is about 3A in filament currents without the 80 tube so the diodes will be pretty hefty.

I also checked all the GND-ing screws on the bypass caps, and on RF cans (though these do not behave this way). And everywhere I could. I poked the GND-ing rivets.
I tried extra decoupling Neg-GND and even cut the cap I use to try short-lead cap in another place. Nope.

One more thing......I have installed the 0.5uF cap in parallel to the filter choke, as per sch. I am not sure it was there when I started to work on the radio. It supposedly should compensate a it of the ripple that comes through the choke as it should come 180 degrees opposite. What if it is not 180 degrees or the value is off....
Then again, the only affected tube is the 1st audio.

And, last thing, I checked the filaments grounding through the centertap, it is Grounded.





Ken:

Yes all shield (they are over every 78, 77 and the oscillator tube), the only unshielded ones (by design) are the AVC 78 tube and three 42, the driver and the output.